The present invention pertains to apparatus and methods for measuring electrical resistance of a thin layer of deposited metal on a substrate and controlling the uniformity of deposited metal during electroplating, electropolishing, or other electrolytic processes that may alter the uniformity and profile of the metal layer. Particular apparatus and methods apply to continuity checking of seed layers and uniformity control of deposited metal layers on partially fabricated integrated circuits during an electrolytic processing.
In order to electroplate metal onto a work piece (or electropolish metal from a work piece), particularly non-conducting or semi-conducting work pieces, a seed layer of metal is typically used to provide a conductive surface through which electrical current can be passed in order to initiate plating. During electrolytic processes, electrical contact to the seed layer on the work piece is made through various mechanisms depending upon the size of the work piece and the criticality of the contact area
For integrated circuit fabrication, the usable plated surface area of a silicon wafer must be maximized so that the highest possible yield of circuit dies can be achieved. In order to increase the usable area, electrical contact to wafers is commonly made by using pins which each make a small surface area (point) contact to the seed layer on the plating surface of the wafer. Preferably, these pins are positioned as far out on the wafer""s edge as possible.
One problem associated with electrical contacts used to supply plating current is that the contacts are exposed to the corrosive effects of plating solutions. In high throughput scenarios such as integrated circuit fabrication, this is particularly problematic. One way to overcome this problem (and to protect the backside of the wafer) is to use an apparatus that has a sealing element between the pins and the plating solution, so that the pins are protected from the plating solution during plating. Although the pins are protected behind the seal, the sealing element itself covers valuable wafer surface area that otherwise would provide usable plating surface.
A problem associated with electrical contacts used to supply plating current to a work piece (e.g a wafer) is variations in resistance among the contacts. Small variations in the resistance of each contact can have a large effect on the thickness distribution of the plated metal on the wafer surface, particularly near the edge of the wafer. Variations in resistance can be caused by poor correction to electrical contacts, build up of plated metal on the contacts, corrosion of the contacts, and the like.
Increasing the usable plated surface area depends not only on the total plated surface area as described above, but also on the quality of the plated surface area, in particular the uniformity of the deposited metal. The uniformity of the plated metal can be directly correlated not only to the uniformity of the plating process, but also to the continuity of the seed layer used to initiate the plating process. Thus, it is important to initiate an electroplating process with a seed layer that has very little if any discontinuous portions.
One way to effectively assess the continuity of the seed layer is by measuring its resistance across the wafer. Currently there are no plating apparatus that allow high-precision (multi-point) resistance measurements of seed layers. Also, with the growing sophistication and miniaturization of integrated circuits, there is a continuing need for highly uniform metal layers on wafers.
What is therefore needed are improved apparatus and methods for continuity checking of a thin layer of deposited metal on a substrate and controlling the uniformity of deposited metal during electroplating, electropolishing, or other electrolytic process that impacts the uniformity of the metal layer. Preferably, such apparatus and methods achieve this goal while increasing the usable surface area of the plated substrate.
The present invention pertains to apparatus and methods for measuring impedance of a thin layer of metal on a substrate and controlling the uniformity of the metal layer during electroplating, electropolishing, or other electrolytic process that impacts the uniformity of the metal layer.
Thus, one aspect of the invention is an apparatus for engaging a work piece during an electrolytic process. Such apparatus can be characterized by the following elements: a cup having an interior region and a lip within the interior region arranged such that lip can support the work piece while the work piece remains within the interior region; a first plurality of electrical contacts arranged about the lip for providing electrical current to the work piece via a metal layer thereon; a second plurality of electrical contacts arranged about the lip for measuring electrical resistance through the metal layer on the work piece; and a cone having a work piece contact surface that fits within the cup""s interior and can contact the work piece in a manner that holds the work piece in a fixed position between the work piece contact surface and the lip. Preferably, a first circuit contains the first plurality of electrical contacts and a second circuit, isolated from the first circuit, contains the second plurality of electrical contacts. Also preferably the metal layer is a seed layer. In a further preferred embodiment, the first and second circuits are at least in part contained within the cup and the cone (wafer holding assembly). In a particularly preferred apparatus, each contact of the first plurality of electrical contacts has its own individually regulated current source.
Particular apparatus and methods apply to deposited metal layers on partially fabricated integrated circuits. In this context, preferably the cup""s lip is sized and shaped to support a semiconductor wafer work piece. Preferably the seed layer and the deposited metal layer are copper; however, other preferred metal layers include cobalt, gold, rhodium, palladium, platinum, tin, chromium, and alloys or mixtures thereof. Also preferably, the cup""s lip comprises a lip seal made from a material that provides a fluid tight seal with the semiconductor wafer when the wafer is held in place by the cone. The lip seal is preferably made of an elastomer, such as a silicone rubber, a fluoropolymer, a butyl rubber, and copolymers or other combinations of these. Specific examples include Chemraz (Green, Tweed, and Co.), Tefzel (Dupont), Sifel (Shin-Etsu), Viton (Dupont), and Kalrez (Dupont). Even more preferably, the first and second plurality of electrical contacts each have electrical contacts that are embedded in the elastomer. In this case, the lip seal includes a material that forms an electrically conductive path from the current supply to the wafer. In a preferred embodiment, the lipseal contains an embedded contact made of a material including at least one of Isocon (Circuit Components, Inc.), conductive polymers (for example metal or carbon impregnated polymers), wires, flat metal springs, and z-conductive polymers. Preferably the electrical contacts are made of metals or alloys that have superior conductive and anticorrosion properties. Alloys of noble metals can be created to maintain conductive and anticorrosion properties while improving mechanical properties (for example strength and fatigue life). Such materials include at least one of berylium-copper, gold-palladium, berylium-copper plated with gold-palladium, Paliney-7 (manufacturer J. M Ney), platinum plated on stainless steel, rhodium plated on stainless steel, and rhodium.
Preferably at least a portion of the cup is made from one or more of the following materials: a plastic, a ceramic, a plastic-coated ceramic, a plastic-coated metal, a glass, a glass-coated metal, a glass-coated ceramic, silicon-oxide coated ceramic, and a composite. Preferably a fluoropolymer is used for the coating of a plastic-coated ceramic or a plastic-coated metal. Preferred ceramics or a plastic-coated ceramics include alumina or zirconia.
Preferably each contact of the first plurality of electrical contacts comprises a resistor. Such a resistor will have a resistance that is large compared to all sources of resistance variation in the electrical current (e.g. plating current). In a particularly preferred embodiment, the resistor is a thick-film resistor made of a material including at least one of ruthenium oxide, platinum-silver, and palladium-silver. Preferably resistors of the first plurality of electrical contacts have an electrical resistance of between about 1 and 20 ohms, and more preferably about 6 ohms. Thick-film resistors are deposited by screen printing methods. They are trimmed to size by laser trimming after deposition, curing, and sealing (for example with glass or silicon oxide). Laser trimming provides accurate matching of conductive paths. Preferably the resistor is positioned at a location between about 2 and 50 mm from the point where its associated contact meets the seed layer, more preferably about 5 mm. Thus, it is desirable to have a large resistance close to the contact to minimize the effect of contact resistance variation.
Methods of the invention allow determination of continuity and resistance of a seed layer on a work piece and electroplating a metal thereon. Therefore another aspect of the invention is a method of electroplating a work piece having a seed layer on its plating surface. Such methods can be characterized by the following sequence: (a) checking the continuity of the seed layer by a resistance measurement using at least a four-point measurement; (b) immersing the work piece into a plating solution; and (c) electroplating a metal onto the work piece""s plating surface. Preferably a first circuit is configured to deliver a plating current to the work piece, and the second circuit is configured to check the continuity of the seed layer, and both the first and the second circuits are are contained, at least in part, in a wafer holder of an electroplating apparatus. Preferably the work piece is a semiconductor wafer.
In one preferred method, during the plating process, the resistance of the deposited metal layer is measured and these measurements are used as a feedback control mechanism to tune the plating current. In a particularly preferred method, plating current is supplied via a plurality of electrical contacts, which are part of the first circuit, each contact having its own individually regulated current source. In this case, the described feedback is used to tune the plating current distribution across the plating surface of the work piece by varying the current supplied to individual electrical contacts. In this way, the uniformity of the plated metal is more precisely controlled.